Vacuum cleaner mechanism responsive to thermal effects of filter bag filling



Jan. 13, 1953 E. P. SENNE 2,625,239

VACUUM CLEANER MECHANISM RESPONSIVE TO THERMAL EFFECTS OF FILTER BAG FILLING Filed May 15, 1948 3 Sheets-Sheet l T0 CUP/PAINT JNVENTOR. [D6/7P P SZ/V/VE/ CONT/904L5 aw fd Jan. 13, 1953 E. P. SENNE 2,625,239

VACUUM CLEANER MECHANISM RESPONSIVEI TO THERMAL EFFECTS OF FILTER BAG FILLING Filed May 15, 1948 3 Sheets-Sheet 2 2 INVENTOR. '%VWWVV 0 44/? P4 fif/v/vi Jan. 13, 1953 E. P. SENNE VACUUM CLEANER MECHANISMYVRESPONSIVE TO THERMAL EFFECTS OF FILTER BAG FILLING 3 Sheets-Sheet 5 Filed May 15, 1948 CONT/FJLLCP F0)? Jam/0w 2470/ 4/? 077 /47? DIV/C15 W rW Patented Jan. 13, 1953 VACUUM CLEANER MECHANISM RESPON- SIVE 'IOv THERMAL EFFECTS OF FILTER BAG FILLING Edgar P. Senn, Rockville Centre, N. Y.

Application May 15, 1948, Serial No. 27,278

Claims.

This invention pertains to the problem of insuring against the undesirable and possibly inadvertent operation of air filter machines, such as domestic vacuum cleaners, beyond a point at which the machine becomes'filled or clogged with dirt or other filtered material to an extent, at which efficiency is imp-aired or the machine dam aged. It contemplates the provision of various thermally responsive control devices which respond to temperature changes occurring in the machine or its parts upon filling or clogging thereof and which, upon response, are capable of actuating means for performing variou remedial, safety or labor saving functions, such as the operation of warning signals or more direct actions such as de-energization of the driving motor, preparation of the machine for removal or replacement of the filter bag by the operator, or the automatic replacement of the filter bag.

It is common knowledge in the art that the efiiciency of machines of the foregoing type becomes impaired as the filter bag becomes progressively more and more full. In point of fact, if the machine is allowed to continue in operation after substantially complete filling, it approaches a clogged condition at which not only isits filtering action rendered almost completely ineffective, but also the clogging may result in serious damage, such as might be caused by overheating due in turn to the consequent diminution of th fiow of cooling air past its driving motor.

It is, therefore, highly desirable, especially in domestic type vacum cleaners more apt to encounter neglectful or forgetful inattent'ion of technically unskilled operators, that means be provided for warning the operator that the undesirable conditicn exists and that remedial action is. required. Better still, it is desirable that there is provided automatically functioning remee dial means, which, for example, actually prevent the operators further use of the machine until such remedial action is taken, or which either automatically replace the filter bagor at least prepare the machine for manual replacement by the operator. Any of the latter automatic fea-. tures will obviously enhance the usefulness efiiciency and general convenience of the machine as by decreasing the labor of the operator and eliminating the necessity of constant inspection.

If means to any of these ends have been su gested heretofore, they have not always been as satisfactory as could be desired in that they do not perform the function with optimum opera.- tional and structural simplicity or with the desired assurance that the means will function without fail whenever the undesirable condition exists.

Now, I have observed two phenomena having thermal efiects which may be utilized to actuate such means in a new and improved manner.

First, I have discovered that most all machines of the general type in contemplation, and specially electrically-driven domestic vacuum cleaners, eX- perience a rather uniform and progressive rise in temperature of the driving motor as the filter bag becomes more and more filled, thi apparently being due to the already-mentioned diminution of the flow of cooling air. The temperature rise occurs primarily in the motor but it is necessarily produces a corresponding rise in the temperature of its surroundings and of the air driven through or around it and out the exhaust outlet. I believe that I am the first to appreciate that this phenomenon is of suflicient magnitude and occurs with sufiiciently non-failing regue larity that it may be used to actuate devices having the purpose already indicated. I have found further that better control of the actuation of such devices can be obtained by utilizing not the change in the absolute temperature of any particular element of the machine but rather the change in the relative or differential temperature between the heated internal parts of the machine and the external or ambient temperature surrounding the machine.

Secondly, noting that the air flowing through the machine may have a cooling effect on any heated member placed in its path, I have observed that the normal decrease in the velocity of flow as the machine fills up may be caused to produce in this cooling efiect variations of sufficient magnitude to actuate thermoresponsive devices to the foregoing ends.

Accordingly, it is the general object of the invention to provide various means responsive to the mentioned thermal effects for performing various warning or remedial functions upon the filling of an air filter machine with filtered material beyond a predetermined limit at which undesirable clogging sets in. It is a further and more specific object to make such provision in domestic type vacuum cleaners. in connection with which the problem is of especial importance because of the greater necessity in machinery designed for usage in that field for foolproof characteristics designed to meet the consequences of neglectful inattention or oversight by technically unskilled operators.

The features of the invention upon which patent protection is sought are set forth in the appended claims. lhe invention itself, together with. further objects and the advantages thereof, will best be understood by reference to the following specification when taken in connection with the accompanying drawings which illustrate a variety of preferred embodiments of the invention as it might be used, for example, in domestic type vacuum cleaners. Throughout the various figures of the drawings like numerals have been employed to designate like parts in View of the similarity of parts among the various embodiments.

In the drawings, the Figure 1 represents in schematic cross section a vacuum cleaner of the type as shown in my copending application Serial No. 9,048, filed February 18, 1948, now abandoned, and illustrating one way in which the invention might be embodied in such a cleaner;

Figure 2 illustrates in cross section the structural details of the thermostatic elements of Figure 1:

Figure 3 represents a schematic illustration like the Figure 1, except that an alternative thermocouple circuit is shown in the place of the thermostatic arrangement of Figure 1;

Figure 4 similarly illustrates schematically the same structure as Figs. 1 and 2 with a thermosensitive reactive bridge circuit in the place of the thermostatic and thermocouple arrangements of the preceding figures;

Figure 5 represents in cross section illustrative structural details of the thermosensitive condensers which might be used in Figure 4;

Figure 6 is similar to Figure 4 but differs in that a thermosensitive bridge circuit composed of resistors is employed;

Figure '7 illustrates one way in which the invention might be employed in a conventional vacuum cleaner to operate a warning light upon filling of the filter bag.

Figures 8 through illustrate arrangements similar to those of the preceding figures but differing in that the temperature change is effected by variations in the rate of air flow past thermosensitive bridge, thermocouple or thermostatic members.

While the invention may be embodied in many types of machines, I have for convenience illustrated in Figure 1 an embodiment in a machine of the type shown and claimed in my aforesaid application. Thus it may comprise a chassis of generally cylindrical cross section adapted to be propelled over a floor upon a pair of wheels (not shown). As in the aforesaid application the chassis may be constructed to comprise a filter compartment l and an air propulsion compartment 2 on the upper and lower sides respectively of a transverse partition 3 which is in operation normally positioned horizontally by virtue of the fact that the relatively heavy air propulsion mechanism (motor and fan units A and 5) brings the center of gravity of the chassis well below the axle 6 upon which the chassis is freely rotatable. For the purpose of introducing air to be filtered into the compartment I, any

suitable means may be provided such as the in let channel 1 which is normally adapted to receive one end of a conventional portable cleaning hose. Immediately beyond the downstream end of the channel 1, air to be filtered passes through the suitable orifices indicated on the drawing into an air filter mechanism (filter bag 8) positioned in the path of incoming air and in such manner that it is readily accessible for replacement purposes. Any suitable structure capable of opening the machine for such replacement may be used; for example, the hinged casing 9 which is adapted to be rotated with bag 8 upwardly and outwardly from the main body of the chassis to an open position in order that the filter bag may be removed. For the purposes of illustrating one particular function of the invention, casing 9 is shown as being biased by coil spring I!) toward such an open positionthis in order that when the casing is automatically released as hereinafter described, the machine will be in such 7 condition that the operator must remedy the filter bag condition before operation can be resumed. During normal operation of the machine and until the bag becomes filled, the casing 9 is held in a closed locked position by means of the latch ll biased by the spring [2 to engage a latch member 13 formed as an extension of the casing 9 below the partition 3.

Within the air propulsion compartment 2 any suitable driving mechanism, such as the motor 4 and the fan 5, may be mounted in such manner that it will draw the air from the compartment I into the compartment 2 through the indicated perforations in the right-hand side of partition 3 and the fioor of casing 9 and thereafter expel it from the machine through a suitable outlet l4all as indicated by the arrows following the flow pattern.

I have observed that with a machine of this type, as well as with out and more conventional types, the motor 4 will rise progressively in temperature as the bag 8 fills progressively. For example, the rise may be enough to increase the temperature of the exhaust air at outlet [4 by amounts of the order of 15 F. between the conditions of a completely empty bag and one which is completely filled. This temperature increase is fairly uniform in its magnitude and appears to occur with regularity such that it may be depended upon for the purposes already indicated. As also indicated, the rise is apparently due to the diminished cooling effect of the air in flowing past the various motor parts. I have found that not only is the temperature increase in the motor fairly constant, but also that the consequent rise in the temperature of the exhaust air coming out of the outlet [4 is similarly uniform and regular and may be depended upon for operation of the means of this invention.

Any of the numerous known thermoresponsive control devices may be employed to take advantage of this phenomenon, for example, thermally expansible and contractible devices such as bellows or bimetallic thermostats. All of these have been exemplified by the thermostatic device 15 which comprises two counteracting bimetallic contact members It and H, the first of which, I6, is positioned directly within the path of the exhaust air flowing through the outlet channel I4. The second, or ll, is positioned externally of the cylindrical tube l8 which forms the channel l4, and, in such position, it is subject to the ambient temperature or the temperature of the surrounding atmosphere. Each of the bimetallic contact members It and I7 is provided with a contact l9 and 28 respectively which protrude through a suitable opening in the tube l8 and into proximity with each other. The thermostatic elements It and I! are so adjusted that when the temperature of the exhaust air has 1 risen to a predetermined degree, proportioned to the extent of filling of the filter bag 8 at which replacement is desired or the stopping of the machine is in order, then the contacts I9 and 20 will be brought together, thereby closing a circuit comprising the electromagnetic relay 2! and the solenoid 22 which operates when energized to withdraw the latch II from engagement with latch member l3. Under these circum- V stances, the motor 4 will be turned off by operation of the opening of the casing 9 may be omitted'if not desired and only the stoppage of the motor may be employed. It will be understood further that the thermostat bimetallic member l6 may be positioned at any point within the machine'at which the desired and useful temperature rise occurs, and'the bimetallic member 11 may be positioned at any point where ambient temperature prevails. Thus, member I6 may be in close proximity to the motor and member I! may be in the path of incoming air in compartment I. They may function together through the medium of suitable interconnecting mechanisms or circuits; that is, it is not entirely necessary that the two thermostats operate by direct cont-act with each other in the manner illustrated.

Illustrative details of the thermostatic device l5 are shown in Figure 2. Each of the thermostats I6 and I! may comprise respectively members of high thermal expansion metal and 26 and members 21 and 28 of metal having a low coefiicient of thermal expansion. Each is mounted in insulating relationship to the other and to the tube I8'by means of the insulating blocks 29 and 30 and the screws 3| and 32 respectively. The separation of the contacts l9 and 20 may be adjusted by a suitable adjusting screw 33 such that the closing of the circuit will occur at the desired degree of temperature rise. To insure positive closing of the contacts l9 and 20, light springs 34 may be so placed that the strips bear against them and they must be compressed before contact is made. It will be understood that the outer external thermostat I! may be so adjusted that contact will be made at some predetermined difference in temperature between the inside and outside of tube l8, i. e., at some desired extent of filter bag filling. The ambient temperature determines the initial shape of both bimetallic members. Since the latter are metallurgically and geometrically identical, the initial spacing between the contacts will be preserved under all temperature conditions.

If desired, a small resistance heater 35 may be wrapped around the bimetallic member 16 and energized by suitable means (not shown). The flow of air over this will tend to reduce its temperature. As the flow is reduced due to the filling of the bag, the temperature of this element will increase for this reason as well as for the reason indicated previously, and thus the thermal efiect which actuates the relay 2| and the'solenoid 22' may be accentuated.

It will be understood that a plurality of thermoresponsive control mechanisms or circuits maybe devised to accomplish the purpose of the thermostatic arrangement of Figure 1. In the Figures 3, 4, and 6 I have illustrated schematically three such circuits. In view of the fact that the machine structure is substantially the same as that of Figure 1, it is indicated more schematically in these three figures and the parts are designated similarly. For that reason, further description of the machine structure is omitted.

In the Figure 3, the circuit is operated by thermocoupl arrangements. One or more thermocouples iIl, depending upon the power requirements, performs the functions of the bimetallic member It and to that end they may be positioned at any point within the compartment 2 at which a usable and workable temperature rise occurs upon filling of the bag. They may, for example, be positioned on the motor 4 or within the path of the exhaust air. In series with the thermocouples 4! there may be provided one or more second thermocouples 4| responsive to the external ambient temperature or to that near the air inlet. The two groups are arranged to actuate any suitable relay 42 in a manner well known in the art of thermocouples. The relay 42 may be caused to perform remedial control functions through a suitable control mechanism indicated by the block 43, which may be a further relay or series thereof adapted to actuate solenoid'latch mechanisms or the motor disconnecting relays similar to those of Figure 1, or a warning signal system such as the light indicated by Figure '7 hereinafter. In order to insure that the thermocouple circuit does not respond to temporary temperature rises or surges unconnected with the filling of bag 8, they may be encased in a sizeable body (not shown) having substantial heat capacity in order that there be sufficient heat inertia to absorb the effect of temporary surges of temperature of short duration.

In Figure 4, a like effect is obtained by the use of a thermosensitive bridge comprising the temperature-sensitive capacitor 44 at some point within the machine subject to the mentioned temperature rise, the capacitor 45 which is sensitive to the ambient temperature as with the devices of the preceding embodiments, and the fixed capacitors it and 41 which complete the bridge and form the standard arms thereof. It will be understood, of course, that the capacitors 44 and 45 are so adjusted that they balance the bridge with the remaining capacitors at temperatures which normally exist when the bag is not in its overloaded condition and when, of course, the bridge is energized from a suitable source of supply 48. When, therefore, the tem perature changes already discussed take place, the bridge will be unbalanced and such unbalance will appear in the transformer 49, the output of which is arranged to actuate a controller 50 of the same nature as block 43 of Figure 3.

Illustrative temperature-sensitive condenser constructions are illustrated by Figure 5 and may comprise thermostatic bimetallic members 5| and 52 arranged to decrease or increase their mutual capacity upon expansion of their respective high expansion members.

Figure 6 accomplishes substantially the same result as Figure 4 and differs therefrom only in that the bridge members are formed of the tern.- perature-sensitive resistors 68 and 6 I, respectively responsive to the machine temperature and the ambient temperature. Standard arm resistors for the bridge comprise the resistors 62 and 83. The bridge is energized by a suitable uni-directional power source such as the rectifier 64 and generator 65. Unbalances are indicated in the neutral arm '56 and relayed through the relay 6! to the controller 68 which is identical with those of the preceding Figures 3 and 4.

Like the Figure 3, the effects of surge temperature rises may be avoided in both Figures 4 and 6 by adding heat capacity.

In the Figure '7 the invention is shown as embodied in a conventional type of cleaner now on the market and generally designated as a tank type cleaner. The cleaner may comprise an inlet it formed in a long cylindrical casing H, a filter bag '12, an air propulsion unit comprising motor 13 and fan Hi, and an exhaust outlet 15 in the casing I I. An ambient temperature-sensitive device 16 and an exhaust temperature-sensitive device i! such as any of those heretofore mentioned may be arranged to actuate a warning signal such as lamp 1%! when the temperature rise already discussed occurs. It will be understood that with the lamp T8 and the devices 16 and H in series with the power supply 19, the warning lamp will be actuated whenever the filter bag becomes filled beyond a predetermined undesirable extent and thereby the operator of the machine will be warned that remedial action is required.

In th Figures 8 through 10, I have illustrated arrangements in which the thermally responsive control devices are responsive to the thermal ef fects of the second of the two phenomenon heretofore mentioned, namely, to temperature changes occasioned by variations in the cooling effect of air flow through the machine on some member as the fiow velocity diminishes with the filling of the machine.

Thermally responsive bridge networks of various kinds may be used and in the Fig. 8, such are exemplified by the thermally responsive resistance bridge having three external resistor arms 80, 8|, and 82, and a fourth resistor arm 83 (preferably of high temperature coefficient of resistance) positioned preferably within the path of the air flow through the machine and preferably at some point not subject to other temperature changing factors, such as a heating influence of the motor 4. Thus, the resistor 83 is shown as being positioned close to the inlet orifice l. The bridge may be energized by suitable means such as the usual domestic power supply source 84 connected to the bridge through a rectifie 85, which may conveniently be of the commonly available selenium rectifier type. derstood that the energization of the bridge in this manner will cause all of the resistor arms and in particular the resistor arm 83 to be heated and that the bridge may be balanced at some set of resistance values corresponding to temperatures within the range of equilibrium temperatures normally existing during operation of the machine when the filter bag is unfilled. The temperature of arm 83 at the balanced condition of the bridge may be adjusted to some workable value which may be substantially affected by changes in air fiow velocity. The bridge will be unbalanced when the resistor arm 83 is at some temperature other than that corresponding to its equilibrium temperature existing when the air is flowing through the machine at the normal velocity existent when the filter bag is relatively empty or at some point less than completely filled.

It will be apparent that as the filter bag becomes more and more filled, the air flow velocity will decrease, thereby diminishing the cooling effect of the air on the resistor arm 83. Consequently, its equilibrium temperature will rise and the bridge will become unbalanced. The unbalance may be reflected through the neutral arm by way of the relay 6'3 which may be arranged to operate the device 63 upon the occurrence of predetermined degree of unbalance in the bridge. Device 68 may perform the function of its counterpart in the preceding figures, i. e., it may perform the remedial functions already described.

In the Figure 9, the use of the principle in connection with thermocouple circuits is illustrated. The circuit may comprise an ambient temperature thermocouple 8B and a thermocouple 87 responsive to thermal effects of air cooling at some point within the fiow path in the machine. For example, the thermocouple 81 may be positioned adjacent some constantly energized (by source 88) resistor 89 or other heating means, the tem- It will be unperature of which will assume varying equilibrium values dependent upon the rate of air flow past its surface. The two thermocouples may be connected in series through the relay and so adjusted that the relay will normally be de-energized when the thermocouple 89 is at a temperature corresponding to the rate of air fiow at conditions of the filter bag less than a predetermined extent of filling. When the filter bag becomes filled to the latter extent, the air flow will consequently diminish and the temperature of the resistor 89 will rise. It will be understood that under such circumstances, the thermocouples may be arranged to energize the relay 90 and that in turn may actuate the device 68 having the function of its counterpart in the preceding figures.

In Fig. 10, a similar arrangement using differential thermostats of the type of Fig. 1 is shown. Here, the ambient and internal temperature bimetallic thermostats 96 and 9| similar to those of Fig. 1, are arranged to control the device 68, which likewise has the function of its counterpart in the preceding figures. A resistor 89 adj acent one thermostat is positioned as in the case of Fig. 9 in the path of the air fiow. The thermostats are so adjusted as to maintain the contacts open except when the temperature of the resistor 89 rises above ambient temperature by a predetermined value corresponding to the diminished degree of air flow existing when the bag has become filled to an undesirable degree.

It will be understood that the foregoing will suggest to those skilled in the art numerous other arrangements which will fall within the true spirit and scope of the invention. Moreover, it should be understood that where in the claims I have used such as expressions as a device for performing a remedial function I contemplate any means for directly remedying or causing an operator to remedy the undesirable condition including such admonitory devices as warning signals, directly acting motor de-energizing devices automatic or otherwise, associated devices with labor saving functions, devices preparing the machine for remedial action by the operator such as the latch release mechanism described or for automatic bag replacement by such means as are shown and claimed in my copending application Serial No. 719,060, filed December 28, 1946, now Patent No. 2,532,642. Where I have used such expressions as a thermosensitive control, I contemplate any of the numerous conventional devices having characteristics which change with temperature and which may be utilized to effect a useful result because of such changes, including thermostats, thermocouples, temperature-sensitive bridges, and other circuits. Moreover, where I have used such expressions as a filled filter bag or the equivalent thereof, I contemplate any predetermined degree, partial or otherwise, at which it might be desirable to perform remedial functions. By ambient temperature I mean broadly the temperature at any convenient point in the surrounding atmosphere or in the machine, which temperature will roughly correspond to that of the inlet air or will otherwise be sufficiently different from the exhaust temperature or other higher temperature as to provide a workable differential with the latter.

I claim:

1. In combination in a vacuum cleaner, electrothermally heated means normally experiencing a change in temperature in response to the filling of said cleaner with filtered material, a device for performing a remedial function for said cleaner upon filling of said cleaner with filtered material to a predetermined degree, and a thermo-sensitive control operable in response to said change in temperature of said first-mentioned means to actuate said device upon filling of said cleaner to said degree.

2. A combination as in claim 1 in which said device is a structure adapted to be opened for the removal of filtered material and in which said control is adapted to efiect opening of said structure upon operation of said control.

3. A combination as in claim 1 including an air propulsion mechanism and in which said device is adapted to de-energize said mechanism upon actuation by said control.

4. A combination as in claim 1 in which said device is a warning signal and in which said control is adapted to actuate said warning signal upon operation of said control.

5. In combination in a vacuum cleaner, electrothermally heated means normally experiencing a change in temperature with respect to ambient temperature in response to the filling of said cleaner with filtered material, a device for performing a remedial function for said cleaner upon filling of said cleaner with filtered material to a predetermined degree, and a thermo-sensitive control operable in response to said change in temperature of said first-mentioned means with respect to ambient temperature to actuate said device upon filling of said cleaner to said degree.

6. A combination as in claim 5 in which said device is a structure adapted to be opened for the removal of filtered material and in which said control is adapted to effect opening of said structure upon operation of said control.

'7. A combination as in claim 5 including an air propulsion mechanism and in which said device is adapted to de-energize said mechanism upon actuation by said control.

8. A combination as in claim 5 in which said device is a warning signal and in which said control is adapted to actuate said warning signal upon operation of said control.

9. In combination in a vacuum cleaner, an air filter mechanism and an air propulsion mechanism normally experiencing a decrease in air flow velocity therethrough in response to the filling of said air filter mechanism with filtered material, a device for performing a remedial function for said cleaner upon filling of said air filter mechanism with filtered material to a predetermined degree, and a thermosensitive control having an element cooled by air flow through said mechanisms and operable in response to change in temperature of said element efiected by decreased air flow through said mechanisms to actuate said device upon filling of said air filter mechanism to said degree.

10. A combination as in claim 9 in which said device is a structure adapted to be opened for the replacement of said air filter mechanism and in which said control is adapted to effect opening of said structure upon operation of said control.

11. A combination as in claim 9 in which said device is adapted to de-energize said air propulsion mechanism upon actuation by said control.

12. A combination as in claim 9 in which said device is a Warning signal and in which said control is adapted to actuate said warning signal upon operation of said control.

13. In combination in a vacuum cleaner, an air filter mechanism and an air propulsion mechanism normally experiencing a decrease in air flow velocity therethrough in response to the filling of said air filter mechanism with filtered material, a device for performing a remedial function for said cleaner upon filling of said air filter mechanism with filtered material to a predetermined degree, and a thermo-sensitive control having an element cooled by air fiow through said mechanisms and an element responsive to ambient temperature, said control being operable in response to change in the temperature difference between said elements effected by decreased air flow through said mechanisms to actuate said device upon filling of said air filter mechanism to said degree.

14. A combination as in claim 13 in which said device is a structure adapted to be opened for the replacement of said air filter mechanism and in which said control is adapted to effect opening of said structure upon operation of said control.

15. A combination as in claim 13 in which said device is adapted to de-energize said air propulsion mechanism upon actuation by said control.

16. A combination as in claim 13 in which said device is a warning signal and in which said control is adapted to actuate said warning signal upon operation of said control.

17. In combination in a vacuum cleaner, a filter bag for removing dirt from air to be filtered, an electric motor for propelling air to be filtered through said bag, said motor normally rising in temperature as said bag becomes filled with collected dirt, a device for performing a remedial function for said vacuum cleaner upon filling of said bag with dirt to a predetermined degree, and a thermosensitive control operable in response to said rise in temperature of said motor and arranged to actuate said device upon filling of said bag to said degree.

18. A combination as in claim 17 in which said device is a structure adapted to be opened for the replacement of said bag and in which said control is adapted to effect opening of said structure upon operation of said control in response to said rise in temperature.

19. A combination as in claim 17 in which said device is adapted to ole-energize said motor upon actuation by said control When the latter is oper ated by said rise in temperature.

20. A combination as in claim 17 in which said device is a warning signal and in which said control is adapted to actuate said signal upon operation of said control by said rise in temperature.

EDGAR P. SENNE'.

REFERENCES CITED The following references are of record in the file of this patent:

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